Immunocytochemical and dye distribution studies of nerves potentially desensitized by injections into the distal interphalangeal joint or the navicular bursa of horses

Abstract

Summary To determine whether the distal interphalangeal (dip) joint directly or indirectly communicates with the navicular bursa (bursa podotrochlearis) and to identify sensory nerves in these synovial structures that might be desensitized by intra-articular injections of anesthetics, Evans blue dye in physiologic saline solution, Luxol fast blue dye with mepivicaine, or commercial latex was injected into the dip joint (5 ml) or the navicular bursa (3 ml) of 152 digits obtained from horses or ponies at necropsy. The digits were frozen, cut with a band saw, and examined for distribution of dye or latex. Of 122 digits that had injections into the dip joint, 120 did not have evidence of a communication between the dip joint and either the navicular bursa or digital flexor tendon sheath. Of 16 digits that had injections into the navicular bursa, 14 did not have evidence of a direct communication with the dip joint. Injection of dye into the dip joint resulted in diffusion of dye and staining of other structures, including the synovial linings of the collateral sesamoidean ligaments and of the distal sesamoidean impar ligament and the medullary cavity of the navicular bone. In addition, a blue tinge was observed in the navicular bursa after dye was injected into the dip joint, suggesting an indirect, and potentially functional, communication between the dip joint and the navicular bursa. Injection of dye into the navicular bursa resulted in staining only of the bursa's synovial lining. Immunocytochemical analysis revealed nerves immunoreactive for the peptidergic neurotransmitters substance P, and calcitonin gene-related peptide located in structures that were stained after dye was injected into the dip joint. Our results suggest that sensory nerves innervating the synovial membranes along the dorsal aspect of the collateral sesamoidean ligaments, sensory nerves of the distal sesamoidean impar ligament, and sensory nerves that directly innervate parts of the navicular bone would, in all likelihood, be desensitized by injections of anesthetics into the dip joint. Therefore, the issue of a communication between the dip joint and the navicular bursa becomes less important during clinical evaluation of syndromes associated with pathologic changes in these 2 synovial cavities (eg, navicular syndrome).